The present invention relates to methods for producing light sources. More particularly, the present invention relates to methods for producing light sources in which light emitted from a light emitting diode (LED) impinges upon and excites a phosphor material, which in turn emits visible light.
White light sources that utilize LEDs in their construction can have two basic configurations. In one, referred to herein as direct emissive LEDs, white light is generated by direct emission of different colored LEDs. Examples include a combination of a red LED, a green LED, and a blue LED, and a combination of a blue LED and a yellow LED. In the other basic configuration, referred to herein as LED-excited phosphor LED, a single LED generates a beam in a narrow range of wavelengths, which beam impinges upon and excites a phosphor material to produce visible light. The phosphor can comprise a mixture or combination of distinct phosphor materials, and the light emitted by the phosphor can include a plurality of narrow emission lines distributed over the visible wavelength range such that the emitted light appears substantially white to the unaided human eye.
One example of a phosphor LED is a blue LED illuminating a phosphor that converts blue to both red and green wavelengths. A portion of the blue excitation light is not absorbed by the phosphor, and the residual blue excitation light is combined with the red and green light emitted by the phosphor. Another example of a phosphor LED is an ultraviolet (UV) LED illuminating a phosphor that absorbs and converts UV light to red, green, and blue light.
Some advantages of white light phosphor LEDs over direct emission white LEDs include, for example, better color stability as a function of device aging and temperature, and better batch-to-batch and device-to-device color uniformity or repeatability. However, phosphor LEDs can be less efficient than direct emission LEDs, due in part to inefficiencies in the process of light absorption and re-emission by the phosphor.
A fundamental problem of optical variation exists at each stage of the manufacture of the phosphor LED. For example, in the manufacture of gallium nitride LEDs, a large number of LEDs are made simultaneously on a single substrate wafer. On this single wafer there can be a variation of peak emission wavelength that can vary from several nanometers to tens of nanometers across the LED wafer. Subsequent deposition of phosphor also adds optical variability to the completed phosphor LED. Process control of phosphor/binder mixture composition, uniformity, and deposition thickness can be problematic for manufacturers trying to achieve uniformity in phosphor LED packages that can be less than one cubic millimeter in volume. Variations in mixture viscosity, mixture homogeneity, raw material homogeneity, and surface tension all combine to make dispensing/depositing a uniform amount of phosphor difficult and costly. These issues combine to produce problems with several phosphor LED attributes such as, for example, color, brightness, angular uniformity, and electrical to optical efficiency.